The number of tax evasion cases being pursued by HM Revenue & Customs has fallen to its lowest level in five years, according to a survey which comes amid a pledge by the Liberal Democrats to crack down on individuals who are not paying their fair share of tax.

Lawyers Pinsent Masons said the number of serious tax evasion cases – with more than £50,000 of suspected tax evasion – had fallen by a quarter in the past year as the government prepares to hand HMRC a general anti-abuse rule from April 2013. Meanwhile the number of raids undertaken in the 2011-12 tax year has risen to 499 from 196 in the 2010-11 tax year.

In 2011-12 there were 3,346 suspected tax avoidance cases, down from the 4,506 in 2010-11, according to data compiled by the law firm.

Phil Berwick, director at Pinsent Masons, said: "HMRC is now prepared to use its strongest anti-evasion measures in cases that would previously have been regarded as quite modest in size".

But, he said, the fall in the number of cases "doesn't really gel with the idea that there is a substantial and growing threat to public spending because of tax evasion".

Lib Dem Treasury chief Danny Alexander said that anyone with assets of more than £1m would face scrutiny by tax inspectors – effectively putting extra 200,000 individuals under the spotlight of HMRC's affluence unit.

Business secretary Vince Cable also told another Sunday newspaper that he wanted to take action against "shady" rich people using offshore tax havens in a "systematic and cynical" way.